Abstract

1. The relationship between body size (W), number of species, and population density (D) was investigated in tropical intertidal communities on the Pacific coast of Panama to address two main questions: (i) does an inverse relationship between body size and average population density like that observed in temperate Chilean rocky intertidal habitats occur in other intertidal communities at other latitudes? and (ii) to what extent does the body size density relationship depend on variation in predation in these habitats? 2. Like their temperate counterparts, intertidal communities in Panama exhibit a log-normal body size distribution: most species are of intermediate size, with smaller numbers of both smaller and larger species. Population abundances of Panamanian species are also log-normally distributed. However, in sharp contrast to the pattern observed in temperate communities, body size of Panamanian species is not correlated with average or maximum population density. 3. Densities of the most abundant species, those that fall on the upper boundary of the body size density distribution, also were not significantly correlated with body size. The 'predicted' upper boundary slope (UBS), obtained from combining the log-normal distributions of body sizes and of population abundances, was much more negative than the observed UBS. 4. Experimental exclusion of a morphologically diverse and species-rich (> 22 species) assemblage of subtidal fishes that forage on intertidal organisms led to a weak but significant negative association between body size and density. A steep negative D W relationship was observed for a subset of these species at sites in Costa Rica, where fish predation is low, suggesting that the differences in the body size density patterns represent regional rather than latitudinal trends. 5. The ability of fishes to suppress the population density scaling is probably linked to the low recruitment of tropical benthic invertebrate species and the 'energy-independence' of these fishes from the intertidal zone. This predator guild can have great impact on all intertidal invertebrates and yet they might be little affected by intertidal prey as they also feed and spend most of the time subtidally.